refs #7421
- simplify README.md to be more use-case oriented and point to https://docs.ghost.org where appropriate
- remove majority of CONTRIBUTING.md content as it now lives at https://docs.ghost.org/contributing
- update adapter guide links in `content/adapters/README.md`
closes#7687
- change location from content/storage or content/scheduling to content/adapters/storage or content/adapters/scheduling
- i have added a @TODO to https://github.com/TryGhost/Ghost/issues/7421 to check if both adapter types needs an update in the documentation